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Shells? Who needs shells?

Exams all done now, so back to the fun of blogging about awesome animals!

So far, the molluscs we have looked at have had some sort of shell, but not all molluscs have taken the shelled path.

Today, and in the next post, we are going to meet some which prefer to be without shells, and are, in my opinion, some of the most beautiful molluscs around: So, without further ado, let me introduce you to todays topic: the nudibranch (from the Latin nudus meaning naked, and the Greek brankhia meaning gills, so literally, “naked gills”)

Berghia coerulescens, a species of nudibranch. Image from wikipedia

This group of animals has a whole lot of awesomeness going on, some of which we will cover today, and then it will spill over into the next post too!

For today, lets take a look at the one in the video, it is called Glaucus atlanticusm and the reason it is awesome for me, is that, despite being a squishy little sea-slug like thing, around 3cm long, it eats these:

Physalia physalis (Better known as the Portuguese Man O’ War) Image from wikipedia

Now, if there is one thing we know about Portuguese Man O’ War, it is that it has a very nasty sting, due to the trailing tentacles being full of cells which can fire toxic darts into its prey (or into an unsuspecting persons leg) (See link HERE for post about Man O’ War)

So how does Glaucus cope with these stings? There is a hypothesis that it releases mucus while eating the tentacles (Yes, because it is small, and the PMOW is much larger, it nibbles its way up the tentacles), which protects its insides from the stings, which can remain active for a period of time after the Man O’ War is dead.

To make it even cooler, it takes some of these stings, and uses them in its own defense. It goes a little something like this:

Glaucus eats the tentacle with the stinging nematocysts within it. Some of these pass through digestion, and end up in growths on the outside of the animal, called cerata. These then become part of the animals defense systems, meaning it can fire them at any attackers.

It has primitive teeth (denticles) which it uses to chomp through its prey, but also to hang onto them

So, this is a fairly cool animal so far, but, it gets even more amazing when you find out some other details about it.

This lovely little animal floats upside down, so its top side is in the water, and its underside is at the surface. It floats because it has a bubble of air in its stomach, but this means it has no way of steering, so it floats around as the winds or currents take it

In common with many fish which are found near the surface (Sharks for example), it uses a form of camouflage which is known as countershading

Its “top” (Which is in the water) is light silvery grey, so it is difficult to spot from below, whilst its “bottom” is a deeper blue, or blue and white, which helps it blend in with the water when seen from above.

I will leave this last image of Glaucus atlanticus for you, and next post we will look at some of the other animals in this family.

9 thoughts on “Shells? Who needs shells?”

You remeark about these animals floating upside down reminds me of something a Prof (expert in C. elegans) said to me. Many (most?) chordates are back to front, treating our spinal cord as though it were in our backs. Have you already covered that in your blog? I think I missed the bit where you moved on from radially symmetrical animals to the bilateral ones.

I can write in a bit of a random, jumbled fashion (My writing mirrors my thought pattern, some might say), but I will try to make sure I explain things a bit more clearly, because I often forget what I have, and have not covered in depth.